In an MRI apparatus (or an MR scanner), an examination object, usually a patient, is exposed within the examination space of the MRI apparatus to a uniform main magnetic field (B0 field) so that the magnetic moments of the nuclei within the examination object tend to rotate around the axis of the applied B0 field (Larmor precession) resulting in a certain net magnetization of all nuclei parallel to the B0 field. The rate of precession is called Larmor frequency which is dependent on the specific physical characteristics of the involved nuclei and the strength of the applied B0 field.
By transmitting an RF excitation pulse (B1 field) which is orthogonal to the B0 field, generated by means of an RF transmit antenna or coil, and matching the Larmor frequency of the nuclei of interest, the spins of the nuclei are excited and brought into phase, and a deflection of their net magnetization from the direction of the B0 field is obtained, so that a transversal component in relation to the longitudinal component of the net magnetization is generated.
After termination of the RF excitation pulse, the MR relaxation processes of the longitudinal and transversal components of the net magnetization begin, until the net magnetization has returned to its equilibrium state. The MR signals which are emitted during the relaxation processes, are detected by means of an RF/MR receive antenna or coil. The received MR signals which are time-based amplitude signals, are Fourier transformed to frequency-based MR spectrum signals and processed for generating an MR image of the nuclei of interest within an examination object.
In order to obtain a spatial selection of a slice or volume within the examination object and a spatial encoding of the received MR signals emanating from a slice or volume of interest, gradient magnetic fields are superimposed on the B0 field, having the same direction as the B0 field, but having gradients in the orthogonal x-, y- and z-directions. Due to the fact that the Larmor frequency is dependent on the strength of the magnetic field which is imposed on the nuclei, the Larmor frequency of the nuclei accordingly decreases along and with the decreasing gradient (and vice versa) of the total, superimposed B0 field, so that by appropriately tuning the frequency of the transmitted RF excitation pulse (and by accordingly tuning the resonance frequency of the RF/MR receive antenna), and by accordingly controlling the gradient magnetic fields, a selection of nuclei within a slice at a certain location along each gradient in the x-, y- and z-direction, and by this, in total, within a certain voxel of the object can be obtained.
Interventional and non-interventional instruments and other medical devices as mentioned above are frequently used during the examination or treatment of an examination object and especially of a local zone or area thereof by means of an MR imaging apparatus. Such instruments or devices are for example catheters, biopsy needles, pointers and other which are used for example for biopsies, thermal ablations, brachytherapy, slice selection and other invasive or non-invasive purposes as mentioned above. Further, RF surface coils, RF pad coils, RF head coils, stereotactic frames and other instruments are also used during MR imaging. For all these and other examinations it is important to precisely position the instrument and especially a certain part thereof (like its tip) at a certain desired location at or within the examination object. This requires that during the positioning of the instrument, the current position of the instrument or an interesting part thereof, especially its tip, is precisely determined and imaged or indicated in the MR image of the related examination object, so that the desired destination at or within the examination object can be reached by an operator.
For this purpose, the above instruments or medical devices are usually equipped with a position marker which can be imaged by means of an MR imaging apparatus in the MR image of the related examination object.
Generally, two different types of such position markers can be distinguished, namely active and passive markers. Active markers as defined in this application are markers which are provided with a cable connection e.g. for being supplied with RF currents for transmitting a local RF excitation field for locally exciting MR signals in the vicinity of the marker, especially in a local volume which at least substantially surrounds or adjoins the marker, in order to detect and image this volume as the position of the marker in an MR image of an examination object, and/or for receiving a local MR signal from the vicinity of the marker, especially from a local volume which at least substantially surrounds or adjoins the marker, and for conveying it to the MR imaging apparatus in order to determine and image the position of the marker on the basis of the received local MR signal.
In the first case of transmitting a local RF excitation field, the resulting local MR signal in the vicinity of the active marker is received either by the marker itself and supplied to the MR imaging apparatus, or by an external (body- or surface-) coil of the MR imaging apparatus, both for determining the position of the marker and for generating a related position indication of the marker in the MR image of the examination object. In the second case of receiving a local MR signal by the marker, the required RF excitation field can be generated by the MR imaging apparatus or again by the marker itself.
Passive markers usually do not need an electrical connection to the MR imaging apparatus as active markers, but can be imaged in an MR image for example by distorting, enhancing or modifying due to their physical properties or due to an own (intrinsic) RF resonance (which is excited by the applied external RF excitation field), the B0 field or the RF excitation field transmitted by the MR imaging apparatus and by this the MR signals emitted by the examination object.
Further, active and passive markers may include a marker material as an own signal source (e.g. fluorine-19), by which MR signals can be excited at a Larmor frequency which is different from the Larmor frequency of the examination object (usually water or fat), wherein by a first RF pulse sequence the position data of the marker material is determined and by a second RF pulse sequence the image data of the examination object is determined, and wherein both data sets are displayed in the form of a common MR image. All these principles enable a position determination and visualization of the active and passive marker, respectively, in connection with the applied gradient magnet fields within the MR image of an examination object.
WO 2006/025001 discloses an MR marker based position and orientation marking system for an interventional instrument, comprising at least three fiducial markers which generate MR signals responsive to an RF excitation generated by the related MR imaging scanner. The three fiducial markers are positioned at the corners of an equilateral triangle, providing a marker assembly which is rigidly connected with the interventional instrument. Each marker includes a small sealed vial containing a magnetic marker material like a fluorine containing material. The first marker includes a first coil having a coil normal oriented in a first direction. The second marker includes a second coil having a coil normal oriented in a second direction different from the first direction and being especially orthogonal to the first direction. These first and the second coil are connected in series to define a first channel and to generate a first quadrature MR receive signal which is connected with a first RF channel receiver. The third marker includes a third coil oriented in the same plane as the first coil of the first marker, however, the third coil is wound and connected with an opposite polarity in relation to the first coil. Further, the first marker includes a fourth coil which is oriented in the same plane as the second coil of the second marker but is wound and connected with an opposite polarity in relation to the second coil. The third and the fourth coil are connected in series to define a second channel and to generate a second quadrature MR receive signal which is connected with a second RF channel receiver. The first and the second quadrature MR receive signals are processed by a position/orientation processor for determining the position and orientation of the marker assembly and by this of the interventional instrument. Alternatively, each marker can be monitored by a separate MR receive channel, and the three received MR signals can be suitably processed to determine position and orientation of the intervention instrument.
In MRM 40: 908-913 (1998) “Integrated and interactive position tracking and imaging of interventional tools and internal devices using small fiducial receiver coils”, by Glyn A., Coutts D., et. al., a method is described of tracking the position of a rigid device within an MR scanner and imaging with the image slice position determined by the current position of the device. The position tracking is performed by means of two or three small solenoid MR receiver coils. Each coil contains a small sample that acts as the fiducial MR visible marker point. The small receiver coils and fiducial assemblies are designed so as to produce sufficient SNR from a fiducial sample small enough to achieve the required localization accuracy while minimizing rotation of the bulk magnetization over the imaging region. The receiver coils are each connected with individual receiver channels comprising each a tune and match circuit and a coaxial transmission line which is short-circuited during transmission of the RF (B1) excitation field by means of a PIN diode and thus behaves as an inductance. By appropriately adjusting the electrical length of the line, the resonant current induced in the receiver coil, and by this the field strength within the receiver coil, can be adjusted to a maximum value, so that the receiver coil can be employed as a local flux amplifier such that when the body magnetization is subject to a 1° rotation, the fiducial sample experiences a rotation several times this figure. If the solenoid receiver coil is oriented perpendicular to the B0 field, the SNR is optimal.
U.S. Pat. No. 6,961,608 discloses an MR imaging system for MR imaging of an interventional device like a catheter, which is inserted into an object and which comprises an RF detection coil at its tip. Further, means for acquiring, together with the application of a gradient magnetic field to the RF detection coil, an MR signal from a vicinity of the RF detection coil excited by an RF magnetic field, and means for obtaining a position of the RF detection coil as the position of the tip through a frequency analyses of the MR signal are provided.
Further, ISMRM 1992, page 104, “Tracking of an Invasive Device within an MR imaging system”, by C. L. Dumoulin, S. P. Souza and R. D. Darrow, discloses that a small RF coil is attached to an invasive device. The RF coil is a simple non-resonant loop with a diameter of smaller than 1 mm which is connected via a coaxial cable to a preamplifier. The RF coil detects MR signals from its immediate vicinity but is insensitive to MR signals coming from more than a few diameters away. By exciting RF relaxation signals in the examination object and generating gradient pulses, the location of the RF coil is determined.
In Proc. Intl. Soc. Mag. Reson. Med. 8 (2000) page 1307, “Active Marker of catheters for MRI guided interventions” by Toennissen et al., a marker comprising an LC resonance circuit for creating a local signal increase in the surrounding tissue is disclosed wherein the signal increase is detected by the related MR head coil and displayed in the MR image. The LC resonance circuit comprises a capacitive element and an inductive element in the form of a spiral coil, a flat coil and a meander coil.
Finally, U.S. Pat. No. 6,574,497 discloses an interventional device in the form of a guide-wire, a guiding catheter, a stent, a tube, a needle and other medical devices that incorporate a fluorine-19 containing material for use as a passive marker as explained above.